A common cutting-off saw essentially includes: a base for placing a workpiece to be cut; a support arm hingedly supported on the base; a driving head mounted on the support arm; and a cutting blade that is able to rotate driven by the driving head. When the support arm is pressed down after the driving head has been started to rotate the cutting blade, the cutting blade will cut the workpiece placed on the base in a desired way.
In order to ensure the operational safety, an upper portion of the cutting blade is enclosed by a fixed guard and a lower portion is enclosed by a movable guard. The structures of a movable guard are respectively disclosed in the patents Nos. ZL200720043502.8 and U.S. Pat. No. 5,893,311. In a typical structure, a geometric center of the fan-shaped movable guard forms a revolute pair with a spindle of the cutting blade; one end of a guard connecting rod is hinged on the body of the movable guard at a predetermined position whilst a straight channel oriented along the center line of the guard connecting rod is formed near the other end of the guard connecting rod; the straight channel forms a sliding pair with a pivot screw disposed near the hinge point of the support arm.
Therefore, during a cutting process, when the support arm is pressed down, the above-mentioned connecting rod mechanism will lead the movable guard to pivot and gradually hide in the fixed guard so as to expose the cutting blade to enable it to cut the workpiece. Moreover, when the support arm is lifted up, the above-mentioned connecting rod mechanism will lose its pulling force on the movable guard and the movable guard will hence pivot in a reverse manner and return to a position fitting with the fixed guard, making the cutting blade completely shielded and avoiding accidental damages caused by an exposed cutting blade. In addition, as the cutting blade is a consumable part, when one needs to replace the cutting blade, the guard connecting rod can be lifted up to move the pivot screw near the hinge point of the support arm to the inner end of the straight channel formed in the guard connecting rod, thus allowing the movable guard to be turned upwards to facilitate the replacement of the cutting blade.
However, the existing structure of the above-mentioned movable guard only has a unidirectional constraint design, that is: when the support arm is pressed down and while the pivot screw near the hinge point of the support arm is located at the outer end of the straight channel formed in the guard connecting rod, the connecting rod mechanism severs to compulsorily drive the movable guard to turn upwards; but there is no constraint to prevent the pivot screw from moving to the inner end of the straight channel; as a result, there is still a possibility for a movable guard with such a current structure to be inadvertently turned upwards and lead to an accident. Thus, strictly speaking, this design cannot meet the requirement that “cutting-off saws shall be so designed that the movable guard will not be accidentally turned upwards during the operation of loading or unloading a workpiece”, specified in part 2 of “particular requirements for cutting-off saws”.
In order to address the aforementioned issue, patents Nos. ZL201020281095.6 and ZL201120033928.1 proposed two technical solutions. In the one disclosed by ZL201020281095.6, a locating pin mechanism is used to restrict the movement of the guard connecting rod. The structure includes a connecting rod assembly consisting of an extension arm, a support arm, an extension-arm connecting rod and a connecting-rod pivot screw. The extension arm is mounted on the support arm. One end of the extension-arm connecting rod in which a straight channel is formed is coupled to the movable guard through a guard connecting rod. The connecting-rod pivot screw is inserted through the straight channel and is located near the other end of the extension-arm connecting rod. The extension-arm connecting rod can rotate around this connecting-rod pivot screw. The locating pin mechanism includes the guard connecting rod and a trough formed in the fixed guard. One end of the guard connecting rod is coupled to the support arm. The connecting-rod pivot screw inserts through the guard connecting rod, the extension-arm connecting rod and the trough formed in the fixed guard, thereby joining these three members together. Therefore, the connecting-rod pivot screw may slides in the trough and thus forms a sliding pair.
The technical solution disclosed by ZL201120033928.1 uses an additional screw to replace the locating pin mechanism of the above-mentioned solution to restrict the movement of the guard connecting rod. In this technical solution, the aforementioned pivot screw is referred to as a first locating screw and the additional screw as a second locating screw. The structure is as follows: an extension arm is mounted on a support arm; the first locating screw is fixed on the extension arm and the second locating screw is fixed near to the first locating screw; one end of the guard connecting rod is hinged with the movable guard near its rotation center; the other end of the guard connecting rod is formed into an arc-shaped elbow; the arc-shaped elbow is hooked on the first locating screw.
Although both the above two technical solutions are capable of restricting the movement of the guard connecting rod to a certain extent and hence reducing the possibility for the movable guard to be accidentally turned upwards during the operation of loading or unloading a workpiece, these structures fail to completely restrict the guard connecting rod. That is, it is still possible for the guard connection rod to slide to accidentally turn upwards the movable guard, and therefore has not completely eliminated the potential safety hazard.